Barry's spot on.
You need to be sure your topping lift is slacked once you've hoisted the main to the correct luff tension. If at any time while sailing in normal breezes you notice the topping lift get taut, loosen it a bit til it's slack.
In the lightest air you can use the topping lift to support the boom when the boom weight distorts sail trim, removing the desired twist. This can be a problem when tacking as the topping lift will chafe/interfere with the leech from time to time.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)